Well, we don't hvae cable tv service, so there is only one line coming out of the box outside, and that goes directly to the modem.

I just did a little experiment to see what would happen: I plugged the modem directly into my computer (not in the router) and was able to get online. It worked for about 10 minutes and then did the exact same thing - data light stopped flashing, I lost all internet connection. So it doesn't look like the router is involved in this problem, unless it's done something to the modem.

Hopefully we'll get it sent in and get a replacement and see if it acts any differently. If the replacement we get acts the same way (both on the router and bypassing the router), would everyone suggest trying to get Comcast to come out and test the line?

Thanks for everyone's help. I really appreciate it!